Along with an increase in the size of a flat panel display (FPD), a flexible display represented by electronic paper has attracted attention. For such a device, various metal films are used for the wiring and electrodes. As a method of forming a metal film, a vacuum film deposition method such as sputtering and vacuum deposition has been widely used, and various circuit patterns and electrodes are formed by photolithography using a photomask.
In recent years, as a wiring/electrode film formation method which is capable of the reduction of the processes required for the pattern formation and is suitable for the mass production and the cost reduction, film formation employing screen printing or an ink jet method has been actively studied. This method forms wiring/electrode film by calcination of conductive fine particles and the like after mixing them with an organic binder, an organic solvent or the like into a paste or an ink and forming the pattern on a substrate directly from the resulting mixture using screen printing or ink jet methods. This method is characteristic not only on the point of the mass and low-cost production being possible due to simpler process than the conventional photolithography, but also on the point of low environmental load because the treatment of the waste and the like in the process of etching is unnecessary. Further, as a low temperature process is possible, this method attracts attention also as a method of forming a film for a flexible display using a plastic or sheet-form substrate.
For production of a metal film by a coating method, the method commonly employed is the method of applying a coating agent obtained by kneading a metal powder with e.g. a paste, on a substrate e.g. by printing, followed by heat treatment. The coating agent used in this method is commonly prepared by taking a preliminarily produced metal powder with high polymer protective colloid etc. and mixing it with a resin etc. (for example, Non-Patent Document 1).
As compared with this method, from the viewpoint of energy saving and simplification of the production process for production of a display panel and various devices, a composition to directly form a metal film from a high-valent metal compound has been desired.
Further, the method for producing a metal powder used for the production of a metal film is roughly classified into a vapor phase method and a liquid phase method.
The vapor phase method is a method of evaporating a metal in a pure inert gas. It is possible to produce a metal powder with little impurities by this method. However, this method requires a large and special apparatus, and accordingly the production cost is high, and the mass production is hardly carried out.
The liquid phase method is a method of reducing a high-valent metal compound in a liquid phase by using ultrasonic waves, ultraviolet rays or a reducing agent. This method is advantageous in that the mass production is easy. As the reducing agent, hydrogen, diborane, an alkali metal borohydride, a quaternary ammonium borohydride, hydrazine, citric acid, an alcohol, ascorbic acid, an amine compound or the like is used (for example, Non-Patent Document 1).
Further, a method has been disclosed to produce a metal powder from an oxide of e.g. nickel, lead, cobalt or copper by using a polyol as a reducing agent (for example, Patent Document 1). However, this method requires a high temperature of at least 200° C. and a reaction time of at least 1 hour. In future, reduction of the total energy for production of various display panels and devices will be essential, and the energy reduction for production of constituting materials to be used is also absolutely necessary. Accordingly, powder production conditions at lower temperature in shorter time, which makes a low temperature process and a short time process possible, have been required.